1. Field of the Invention
This invention relates to a process for the formation of a circuit of a printed circuit board, and especially to a process for the formation of a conductive circuit pattern of a printed circuit board, which is well suited for forming a microcircuit pattern at a high density.
2. Description of the Related Art
As a process for forming a conductive circuit pattern of a printed circuit board, it has heretofore been the common practice to use a copper clad laminate as a starting material, to provide an etching resist at circuit-forming areas and then to remove a copper foil at areas other than the circuit areas by etching.
Etching, however, proceeds not only in the direction of the thickness of the circuit but also in its widthwise direction, so that the cross-sectional profile of the circuit is not rectangular but is trapezoidal with its width broader on a side of a substrate. It has therefore been difficult to obtain a sufficient circuit cross-sectional area when the wiring density becomes high and the width of each conductive strip becomes closer to the thickness of the circuit. Further, the dimensional accuracy of a circuit is determined by the extent of variations in etched quantity, leading to the problem that it is difficult to form a microcircuit pattern at high accuracy.
To form such high density wiring, a pattern plating method is therefore used instead of the usual etching.
According to the pattern plating method, a copper clad laminate is used as a starting material. A plating resist is applied on a copper foil at areas where no circuit is to be formed. The copper foil is plated at areas not covered with the plating resist, namely, at circuit-forming areas, whereby a conductive circuit pattern is formed. Thereafter, the plating resist is removed and the base copper is removed at the areas other than the circuit by etching, whereby an independent circuit is formed.
Since the conductive circuit pattern is formed in exactly the same pattern as the pattern of the plating resist in this method, the accuracy of formation of a circuit width is determined by the accuracy of formation of the plating resist. It is therefore possible to form a microcircuit pattern by using a photo-imagable plating resist and forming a resist pattern by a lithographic process.
For such a pattern plating method, it is essential to have a plating resist sufficiently adhered on a substrate so as to prevent delamination or blistering in a plating step and also to completely peel of the plating resist after completion of the plating. In other words, mutually contradictory characteristics are required, including high adhesion up to the plating step and good peelability in the resist peel-off step. When the formation of a conductive circuit pattern is conducted by electroless copper plating in particular, the resist tends to peel off or blister in the plating step so that improvements are required in the adhesion of the plating resist.
As methods for improving the adhesion of a plating resist to prevent peeling or blistering in a plating step, there are methods disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) Nos. SHO 64-13794 and HEI 1-261888.
To strip off a resist, a chlorinated organic solvent such as dichloromethane or an alkaline resist stripper is generally used depending on the properties of the resist. As a result of an improvement in the adhesion of a resist by the method described above, the above stripping treatment cannot completely remove the resist so that residual resist may still remain locally in a state adhered on the base copper foil at areas other than the circuit areas.
The remainder of such resist residue leads to a failure in removing the copper at such non-circuit areas in the subsequent etching step, thereby causing defects such as short circuits and insufficient inter-strip spacing.
To remove any adhered residual resist, Japanese Patent Application Laid-Open (Kokai) No. SHO 60-206190 therefore discloses a method in which any remaining photoresist is dipped in a strong acid and then brushed off by an acid-resistant brush.
Use of a mechanical method such as brushing for the removal of resist residue as in the above-described conventional technique, however, tends to damage, peel off or cut off the microcircuit pattern.